Digital audio content appears in many different instances, including for example music and movie files. In most instances, an audio signal is encoded for purposes of data-rate reduction, so that the transfer or delivery of the media file or stream consumes less bandwidth and is faster, thereby allowing numerous other transfers to occur simultaneously. The media file or stream can be received in different types of end user devices where the encoded audio signal is decoded before being presented to the consumer through either built-in or detachable speakers. This has helped fuel consumers' appetite for obtaining digital media over the Internet. Creators and distributors of digital audio programs have several industry standards at their disposal, which can be used for encoding and decoding audio content. These include Digital Audio Compression Standard (AC-3, E-AC-3), Revision B, Document A/52B, 14 Jun. 2005 published by the Advanced Television Systems Committee, Inc. (the “ATSC Standard”), European Telecommunication Standards Institute, ETSI TS 101 154 Digital Video Broadcasting (DVB) based on MPEG-2 Transport Stream in ISO/IEC 13818-7, Advanced Audio Coding (AAC) (“MPEG-2 AAC Standard”), and ISO/IEC 14496-3 (“MPEG-4 Audio”), published by the International Standards Organization (ISO).
There is an ever increasing variety of end user devices for the playback of digital audio, including desktop computers, laptop computers, portable handheld devices (e.g., smartphones), home televisions, and in-vehicle media systems. These devices have different analog signal paths, speakers and acoustic environments. Also, the dynamic range of an audio signal varies between different programs. In addition, producers (including creators and sometimes even distributors) of digital audio programs often wish to increase the average loudness of their programs, by digitally modifying an audio signal so that its average loudness is higher by several dB. Doing so however also requires that the peak levels of the resulting audio signal be reduced in order to avoid clipping (which leads to undesirable audible distortion). This is achieved using dynamic range control (DRC), which compresses the highs and lows of the audio signal so that the resulting audio signal can fit within a narrower envelope (thereby avoiding clipping). All of these factors however create an issue in that loudness as perceived by the end user can vary significantly, both across different end user devices and between consecutive programs on the same device, leading to an unpleasant user experience during playback.
A software tool such as the SoundCheck™ program by Apple Inc. automatically adjusts the playback volume of songs to hopefully yield the same perceivable loudness, so that for example a recent pop recording that has a smaller dynamic range but higher average loudness is turned down, as compared to an older song from the 1970's. Also, an audio program can include a metadata portion that is associated with the encoded audio signal and which describes the associated audio signal. The metadata can include information that is used by software in the end user device to control for example the dialogue level, DRC, and any downmixing of the decoded audio signal, so as to change the consumer's experience during playback.